Add dunfell support

[mirzak]

Should be based on components of the Mendel 5.0 release

https://coral.ai/news/updates-07-2020

Weirdly, I can not see this release in the release notes

Things that I know need to be done (not a complete list):

• [ ] Fetch libedgetpu from github location (#16)
• [ ] Update U-Boot recipe to 2019.04
• [ ] Update GPU patches for newer versions

[mirzak]

Wont fix, dunfell is almost out of LTS support now. Next release target is Mickledore (4.2 )